CuInSe.sub.2 (CIS) and Cu(In, Ga)Se.sub.2 (CIGS) are compound semiconductors (of chalcopyrite structure) comprising at least one element from each of groups Ib, IIIb, and VIb. Thin-film solar cells using these semiconductors for the light-absorption layer have been reported to achieve a high energy conversion rate, which does not deteriorate due to irradiation. Throughout this specification, "groups Ib, Ib, VIb, II, IIa, and IIb" refer to "groups 1B, 3B, 6B, 2, 2A, and 2B" of the periodic table of elements according to the old IUPAC recommendation before 1985. Thus, group IIIb refers to the group B, Al, Ga, etc.
Conventional CIS or CIGS solar cells are generally made by forming an n-type semiconductor layer by chemical bath deposition on a CIS film or a CIGS film formed by vapor deposition or selenization.
FIG. 18 illustrates an example of a conventional CIS or CIGS solar cell. A conventional solar cell 1 comprises a substrate 2, a rear electrode 3 deposited on the substrate 2, a p-type compound semiconductor layer 4 made of a CIS film or a CIGS film, an n-type semiconductor layer 5, a ZnO film 6, and a transparent conductive film 7. The n-type semiconductor layer 5 is made of Zn(O, OH, S).
In the conventional solar cell 1, a pn-junction is formed between the p-type compound semiconductor layer 4 made of a CIS film or a CIGS film and the n-type semiconductor layer 5. This pn-junction has a large influence on the energy conversion ratio. Several methods have been proposed for forming the n-type semiconductor layer 5 in a manner that improves the junction formed between the p-type compound semiconductor layer 4 and the n-type semiconductor layer 5.
However, one of the mains problems in such a conventional solar cell 1 is that the structures of the p-type compound semiconductor layer 4 and the n-type semiconductor layer 5 are completely different, so that defects near the junction are abundant.